This invention relates to a unique method of minimizing the detrimental effect of scroll or screw compressors flooded starts.
Scroll compressors are widely utilized in refrigerant compressor operation. One known type of scroll compressor includes compression elements and an electric motor housed within a sealed compressor shell. A quantity of lubricant is also received in the compressor shell. In such compressors, the refrigerant passes over the motor on its way to the inlet of the compression elements, cooling the motor.
In a scroll compressor a pair of scroll members have wraps which interfit with each other to define compression chambers. When rotated in a forward direction, a normal compression process occurs in which refrigerant is trapped between the wraps and compressed towards a discharge port.
At startup the oil, located in a compressor sump, may contain a quantity of liquid refrigerant. At startup the sump and motor are cool, and preheating does not occur. The presence of the unpreheated oil/refrigerant mixture in the oil sump has undesirable effects.
The problem is particularly acute in refrigerant systems for intermodal transport, here refrigeration takes place in large containers used to transport fruit or other food products over long distances. Inter-modal refrigerant containers may be initially shipped on a boat, transferred to a train, and then transferred to trucks. Refrigeration must be maintained throughout the entire trip. The container may then be returned to a remote location for storage. Thus, during its life cycle a container refrigeration system may often be shut down for long periods of time.
The problem can become especially severe during cold starts. When the refrigeration system is shut down, under certain ambient conditions, large portions of liquid refrigerant contained in the system can migrate to the compressor sump. Thus oil located in the compressor sump can be diluted with liquid refrigerant. This is undesirable because on startup it can result in liquid refrigerant passing through the scroll elements. This can result in bearing and scroll element damage. Also oil dilution by the refrigerant may lead to lower lubricant viscosity, which is also detrimental to bearing life.
Crank-case heaters have been proposed to address this problem. Crank-case heaters are turned on prior to startup of the compressor to preheat the sump of the compressor and begin to address the problems discussed above.
However, crank-case heaters require a long time to heat the oil and refrigerant mixture. Thus, the heaters must be turned on for a relatively long period of time before the compressor can be started. In addition, a crank-case heater is relatively expensive, and presents its own reliability problems.